Organic carboxylic acids have long been known to react with inorganic particulate minerals, including alumina and other aluminum oxyhydroxides, to produce fillers that disperse more easily than the base inorganic particulate in paints, plastics, rubbers, adhesives, caulks and other composites materials. For example, U.S. Pat. No. 4,420,341 (Ferrigno, 1983) describes surface modified fillers with improved properties that comprise a particulate mineral, an organic acid, an antioxidant, and a liquid agent. U.S. Pat. No. 4,283,316 (Bonsignore 1981) describes the modification of alumina hydrates with liquid fatty acids to make them compatible with thermoplastic polymers, and U.S. Pat. No. 4,191,670 describes the treatment of mineral fillers with a mixture of saturated and unsaturated aliphatic acids. However, the above cited patents do not specifically address the technical challenges of working with nanoparticle materials that have at least one dimension of less than 100 nm. [Also see for example “Filler surface modification with organic acids,” Plastics, Additives and Compounding Volume 2, Issue 12, December 2000, Pages 26–29, Elsevier Science and “Thermochemical study of Adsorption Behavior of Polyacrylic Acid on Alumina Powder Surface” Uchida et. al., Key Engineering Materials, Vols. 161–163 (1999), p. 133–136]
Nanoparticle materials (that have at least one dimension less than 100 nm) offer unique properties not available with corresponding macro- or micro-fillers. For example, large improvements in barrier, mechanical, or thermal properties have been demonstrated in nylon-clay nanocomposites containing only a few volume percent of the clay nanofiller. With conventional fillers, volume fractions of 30%–40% are required to achieve similar improvements. Improvements in the nylon nanocomposites included a doubling of the tensile modulus and strength for clay loadings as low as 2 vol. % inorganic. In addition the heat distortion temperature of the nylon nanocomposites was increased by up to 100° C. [see for example, U.S. Pat. No. 4,739,007; Kojima et al. “Mechanical properties of nylon 6-clay hybrid”, J. Mater. Res., (1993), 8, 1185–1189; Usuki, et al. “Synthesis of nylon 6-clay hybrid”, J. Mater. Res., (1993), 8, 1179–1184; Yano et al. “Synthesis and properties of polyimide-clay hybrid”, J. Polym. Sci. Part A, (1993), 31, 2493–2498]. Also, particulate fillers have long been known to impart desirable properties to a variety of polymeric materials. For example, mica increases the stiffness of phenol-formaldehyde plastics (A. King “Application of Fillers” in Plasticizers, Stabilizers, and Fillers, P. D. Ritchie ed, Iliffe Books, London, 1972.) Plate-like fillers have been known to improve the barrier properties of their composites (A. A. Gusev and H. R. Lusti, “Rational Design of Nanocomposites for Barrier Applications”, Advanced Materials, 2001, Vol. 13(21), 1641–1643). Many other polymers and nano-fillers have been studied in addition to nylon-clay nanocomposites.
As evident from the large amount of research carried out in the field on nanomaterials and nanocomposites over the past decade [see for example: Dagani R., “Putting the “Nano” into Composites”, Jun. 7, 1999, C&EN, 25–27; Thayer A. M., Nanotech offers some there, there”, Nov. 26, 2001, C&EN, 13–16; and Beall, G. W., “New Conceptual Model for Interpreting Nanocomposite Behaviors” in “Polymer-Clay Nanocomposites”, Pinnavaia T. J. and G. W. Beall, Eds., 2001, John Wiley & Sons, Incorporated, Chichester, England, pages 267–279], the methods and principles used for processing traditional macroscopic and microscopic fillers cannot be readily translated into nano-sized materials. Nano-sized filler materials are substantially more challenging to process than common macro- and micro-fillers because their extremely high surface area causes problems of high viscosity when mixed with liquids or polymers and problems of high oil absorption when mixed in resins and oligomers. Foaming or defoaming are other common side-effects of amphiphilic nano-fillers. Agglomeration during drying or processing is another common problem of nano-sized fillers.
In many cases, it is advantageous to provide inorganic particles with an organic surface modification. Modifying the surface of particles that are added to a polymer matrix to form a composite can improve the wetting of the particles by the matrix and improve the dispersion of the particles in the matrix, thereby improving such properties of the composites as strength, toughness, and the ability to act as a barrier. Surface modifications can also improve the adhesion between the particles and the polymer, thereby improving the load transfer and the mechanical properties of the composite. For example, U.S. Pat. No. 4,091,164 teaches the modification of kaolin clays by mixing the clay particles with block copolymers of ethylene oxide and propylene oxide and then melting the polymers so that they adhere to the clay particles. Surface modifications have also provided particulate fillers the ability to bond with a matrix, as is described in e.g. U.S. Pat. No. 3,901,845, which teaches coupling of a mineral filler with a nylon matrix by an aromatic compound having a carboxyl group and a hydroxyl or amine group. PCT application WO 00/09578 and U.S. Pat. No. 6,369,183 B1 also teach surface modification of a filler followed by coupling of the filler to an organic matrix.
Boehmite is a particularly useful material for polymer nanocomposites because agglomerates of nanosized primary particles are readily available at low cost and high purity, and the commercially available agglomerates can be processed into primary particles. The surface of the boehmite can be modified with organic molecules via bonding between carboxylic acid groups and the boehmite surface. Also, nanoparticles can be made which have a boehmite surface layer and a non-boehmite inner core. For example, alumina nanoparticles can be coated with a boehmite surface layer and then modified with organic acids.
Some of the challenges of working with nano-sized fillers have been addressed in the past. A particularly vexing problem is the tendency of nanoparticles to strongly stick together, forming agglomerates of many primary nanoparticles that can be much larger and therefore less useful than the primary particles. Even worse, even if the agglomerates are broken apart, the particles tend to phase separate and re-agglomerate when they are added to a polymer. For example, U.S. Pat. No. 5,935,275 (Burgard, Nass, Schimdt, 1999) reports methods to prepare weakly agglomerated nano-sized particles, U.S. Pat. No. 6,190,731 (Tecle, Berhan, 2001) reports methods to make isolated ultrafine particles by encapsulating a highly dispersed colloidal suspension with an encapsulant material, and U.S. Pat. No. 6,537,665 (O'Connor, Nehring, Russell, 2003) reports a particulate powder adapted for dispersion in organophilic solvents that comprises a powder particulate material, a first coating on the particulate material that contains a reagent that causes the surface of the particles to be reactive to an organic acid derivative, and a second coating that covers the first coating and contains an amphiphilic surfactant and an organic acid derivative.
More specifically, methods have been reported to prepare organic surface-modified aluminum oxyhydroxide particles of nano-sized dimensions [U.S. Pat. No. 6,369,183, Cook and Barron et al. in Chem. Mater. 1996, 8, 2331–2340, Landry, Barron et al. in J. Mater. Chem. 1995, 5(2), 331–341; Cook, Barron et al. in Chem. Mater. 1997, 9, 2418–2433, Barron et. al. in Chem. Mater. 2000, 12, 795–804, Barron et al. in Macromolecules, 2000, 12,795–804, and Obrey and Barron in Macromolecules, 2002, 35, 1499–1503.]. However, these reported methods produce materials with different (and less useful) properties than the materials prepared according to the present invention. These references have focused on the dispersion of boehmite nanoparticles for the purpose of processing them in aqueous solutions. Typically, these dispersed boehmite nanoparticle solutions are used as precursors in the production of ceramic materials. The organic acids are used primarily to disperse the nanoparticles into water and there is no need to design the nature of the organic modifying groups or use multiple acids to achieve a more specific surface chemistry so that they interact correctly when they are added to a polymer to form a nanocomposite. These references do not teach the production of surface-modified boehmite nanoparticles that have two or more organic acids. Using multiple (two or more) acids, in contrast to a single acid, yields many more possible types of surface chemistries that allow one to tailor the solubility, surface-graft density, and the length of attached organic groups, and to vary combinations of the length of attached organic groups to provide specific nanoparticle-polymer interactions. For example, one organic acid can be used to tailor the surface properties to prevent agglomeration and/or phase separation, while another can be used to attach other useful functionalities to the particle. An additional advantage of using multiple acids for surface modification is the possibility to introduce on the surface complementary functional groups such as a catalytic site and a co-catalyst. Particularly useful applications of nanoparticles functionalized with two acids have been reported in commonly owned U.S. Pat. No. 6,933,046, issued Aug. 23, 2005; U.S. Pat. No. 6,986,943; issued Jan. 17, 2006 and U.S. Pat. No. 6,887,517, issued May 3, 2005.
One useful aspect of the present invention is a method for producing surface modified nanoparticles in which some of the organic modifiers are large molecules, having a molecular weight greater than or equal to 500 Daltons. Others have described the difficulties associated with trying to attach large molecules (defined as larger than 500 Daltons) to nanoparticles [U.S. Pat. No. 5,593,781 Nass, Schmidt and Schmitt]. The present invention teaches a method for partially modifying nanoparticle surfaces with some small molecules at a concentration sufficient to provide for dispersion in appropriate solvents and subsequent addition of large molecules directly to the nanoparticle surface. Similarly it has been difficult to react hydrophilic inorganic particles such as boehmite with hydrophobic molecules that are not water-soluble. The present invention provides a means to modify boehmite particles with hydrophobic carboxylic acids.
Additionally, previous works have not taught the significance of limiting the amount of carboxylic acid used to modify the surface of boehmite so that the acid does not chemically alter the boehmite material. In the present invention the ratio of acid to boehmite is limited to prevent unwanted degradation of the boehmite. The following references and examples 1, 2 and 3 (herein) will illustrate this principle.
U.S. Pat. No. 6,224,846 (Hurlburt, Plummer, 2001) teaches a method of making a dispersible alumina by reacting a slurry of boehmite in water with the salt of an organic sulfonic acid. Sulfonic acid derivatives are highly hydrophilic and therefore are often not desirable in many coating and plastic applications. Therefore, there is an advantage in using hybrid nanoparticles that do not contain sulfonic acids. Hurlburt and Plummer do not teach the reaction of multiple sulfonic acid groups with boehmite. U.S. Pat. No. 6,224,846 also reports a process for preparing metal oxide particles modified with sulfonic acids.
U.S. Pat. No. 4,676,928 (Leach, Decker, 1987) reports a process for producing a water dispersible nano-sized alumina by forming an aqueous alumina slurry from an uncalcined alumina and admixing a monovalent acid to produce an alumina slurry-acid composition having a pH of 5.0 to 9.0, and then aging the composition at a temperature higher than 70° C. The alumina content of the slurry is reported to be 9 to 15% by weight. Monovalent acids disclosed in U.S. Pat. No. 4,676,928 include nitric acid, hydrochloric acid as well as, formic acid, and acetic acid. However, most of the examples show the use of nitric acid (an inorganic acid) with the exception of Example 11 of U.S. Pat. No. 4,676,928, where either acetic acid or formic acid is used. In this example, the alumina slurry was aged at 190° F. for a month to obtaining a water dispersible sol. Leach and Decker only teach the use of a single organic acid, and they teach that it is preferable to use nitric acid, which is not a carboxylic acid.
U.S. Pat. No. 5,593,781 (Nass, Schmidt, Schmitt, 1997) reports a method of manufacturing surface-modified nanometer size ceramic powder by suspending the powder in water or an organic solvent, adding an organic compound which interacts or reacts with groups present on the surface of the ceramic powder, then removing the water or organic solvent. The nanometer-sized ceramic powders of U.S. Pat. No. 5,593,781 include aluminum oxyhydroxides and the modifying organic groups include carboxylic acids. This patent does not teach the use of multiple carboxylic acids and does not teach or recognize the benefits of using multiple carboxylic acids.
U.S. Pat. No. 6,030,599 (Noweck, Schimanski, Meyer, 2000) reports a process for producing water-dispersible alumina hydrates by hydrolyzing and condensing an aluminum alcoholate in water in the presence of a polymerization inhibitor selected from a group of compounds that comprise organic and inorganic acids. Similarly, U.S. Pat. No. 4,211,667 (Yamada, Yoshihara, Ishida, and Sato, 1980) reports a process for producing an alumina sol by neutralizing a water-soluble aluminum salt to produce an alumina gel and then subjecting the gel to hydrothermal treatment in the presence of a single monovalent organic acid. In contrast, the present invention teaches a process that utilizes pre-formed aluminum oxyhydroxide particulate materials. Since aluminum oxyhydroxide powders that are useful for this invention are commercially available in large quantities and at reasonable price, it is advantageous in many situations to start from the pre-formed ceramic powder, rather than have to carry out the hydrolysis and condensation of aluminum alcoholates or aluminum salts as described in U.S. Pat. Nos. 6,030,599 and 4,211,667.
Apblett et al. [Mat. Res. Symp. Proc. Vol. 249 1992] reports the formation of carboxy substituted particles from the reaction of pseudoboehmite and a single carboxylic acid.
Landry et al. [J. Mater. Chem. 1995, 5(2), 331–341] describe the reaction of [Al(O)(OH)]n with a single carboxylic acid (HO2CR) to form species [Al(O)x(OH)y(O2CR)z]n where R=C1–C13 and 2x+y+z=3. No mixed species, where different R groups were substituted on the alumoxane, were reported.
U.S. Pat. No. 4,983,566 (Wieserman, Karl, Cross, Martin, 1991) reports materials comprising a metal oxide/hydroxide particle that have been reacted with a prefluorinated organic acid, including multiple perflourinated carboxylic acids. Wieserman et al. do not teach the use of organic carboxylic acids having at least one C—H bond.
U.S. Pat. No. 6,369,183 (Cook, Barron, and others, 2002) reports thermoset polymer compositions formed by reacting amine, hydroxyl, acrylic and vinyl substituted carboxylato-modified boehmite with low molecular weight polymer precursors to form a cross-linked network in which the particles are covalently linked to the polymeric matrix. This patent reports methods for preparing these substituted carboxylato-modified boehmite particles (called carboxylato-alumoxanes). However, the materials prepared according to this patent are different (i.e. have different properties) from the materials prepared according to the present invention. In Examples 2–14 of U.S. Pat. No. 6,369,183 carboxylato alumoxane are prepared by mixing boehmite with a carboxylic acid in a ratio of total aluminum atoms of the boehmite to carboxylic acid ranging from 1:4 to 3:2. The use of such high ratios of acid to boehmite attack and in some cases dissolve the boehmite particles, as will be described in more detail herein In contrast, the method of the present invention limit the amount of acid used to avoid, minimize or control the dissolution of boehmite nanoparticles. The dissolution of boehmite can be observed by the disappearance of the typical peaks of boehmite in the XR-D of the product and appearance of new peaks (as shown in Examples 2B and 3B, herein).
Examples 1A, 2A, and 3A and comparative examples 1B, 2C, and 3B, herein show the differences in properties of materials prepared according to this invention and materials prepared according to the methods of U.S. Pat. No. 6,369,183 As evident from the X-RD of the products, partial or complete dissolution of the boehmite particles occurs when high concentration of acids are used according to U.S. Pat. No. 6,369,183. Additionally, in contrast to the materials of this invention, the carboxylato alumoxanes prepared according to U.S. Pat. No. 6,369,183 B1 can be separated in two or more fractions of different chemical compositions that have different solubility properties. Furthermore, when using acids that contain polymerizable double bonds (such as methacrylic acid in Example 14 of U.S. Pat. No. 6,369,183 B1) the product contains polymerized methacrylic acid.
The synthesis of carboxylato alumoxanes from boehmite and carboxylic acids is also reported in Cook, Barron et al. Chem. Mater. 1996, 8, 2331–2340, Landry, Barron et al. J. Mater. Chem. 1995, 5(2), 331–341, Cook, Barron et al. Chem. Mater. 1997, 9, 2418–2433, Barron et. al. Chem. Mater. 2000, 12, 795–804, Barron et al. iMacromolecules, 2000, 12, 795–804], and Obrey and Barron Macromolecules, 2002, 35, 1499–1503. All the materials reported in these papers are prepared by reacting boehmite with 1–40 molar equivalents of an organic carboxylic acid. As detailed above and demonstrated by the comparative examples 1A versus 1B, 2A versus 2B and 3A versus 3B, the properties of the materials obtained with these prior art methods are different from the properties of the materials obtained according to the method of present invention.
Example 48 in U.S. Pat. No. 6,369,183 reports the production of a modified pseudoboehmite particle employing methoxyethoxyacetic acid (MEEA) and sebacic acid. The boehmite is first reacted with MEEA to form a “MEEA-alumoxane” (e.g. a boehmite particle that has been reacted with methoxyethoxyacetic acid) and then the reacted product is stirred for several hours with sebacic acid. The reference teaches making the materials using a level of acid greater than 1:1 (aluminum to acid mole ratio). It does not teach using multiple organic acid beohmite nanoparticles at a loading level (2:1 aluminum to acid or at lower acid levels) that results in the materials described in the present invention. Furthermore the product of Example 48 of U.S. Pat. No. 6,369,183 is an insoluble cross-linked material. The sebacic acid used in that example has two reactive carboxylic acid groups and therefore reacts with more than one particle forming a continuous network of cross-linked particles. In contrast, the methods of this invention produce surface-modified nanoparticles that are not cross-linked with other particles and therefore retain their ability to disperse in water, solvent, polymers, resins and other systems.
There is a need in the art for surface-modified boehmite nanoparticles that are not cross-linked or otherwise aggregated and which exhibit useful rheologic properties including dispersibility in water, water/solvent mixtures, and selected organic solvents as well as in polymers, resins and other materials. Additionally, there is a need in the art for surface-modified boehmite nanoparticles in which the density of organic groups on the surface is selectively controlled to adjust and control the surface properties of the particles.